1. Field of the Invention
The present invention relates to a new current source inverter for driving AC motors such as three-phase induction motors and synchronous motors. More particularly, this invention is concerned with a current source inverter which can markedly reduce stresses on the high voltage thyristors and the high voltage motor terminals, and which can operate at a high efficiency over a much wider frequency range with much lower voltage stresses on the motor terminals and the power devices.
2. Description of the Prior Art
Current source inverters (CSI) are well known in the art. The most widely used CSIs include an auto-sequentially commutated inverter (ASCI) (K. P. Phillips, "Current Source Converter for AC Motor Drives," IEEE Trans. Ind. Appl., Vol. IA-6, pp. 679-683 Nov./Dec. 1972). The ASCI has many advantages such as ruggedness, fuseless protection, full four-quadrant operation, etc., whereas it has quite a few drawbacks in that it requires: (1) a number of large high voltage capacitors: (2) high voltage thyristors and power diodes; (3) high voltage motor terminal stresses (i.e., surges); and (4) a limited range of operating frequencies. Thus, the ASCI is usually designed by compromising high voltage stresses and operating range. In spite of such limitations, the ASCI has been prepared simply because no other CSI comparable with the prior art ASCI has been developed. Further disadvantages with respect to the prior art ASCI will be discussed hereinafter with reference to the accompanying drawings.